This invention relates to a color cathode-ray tube having an internal magnetic shield, and more specifically to a color cathode-ray tube having an internal magnetic shield which is so constructed that an electron beam is less affected by external magnetic field such as terrestrial magnetism from the time it is emitted from an electron gun to the time it strikes a fluorescent layer through a shadow mask so as to provide a display image of high color purity.
A color cathode-ray tube generally has an evacuated glass envelope (bulb) comprising a panel portion located at the front and having a face plate of large diameter, a neck portion of small diameter located at the rear, and a substantially funnel-shaped funnel portion connecting the panel portion and the neck portion. In the panel portion, a fluorescent layer is formed on an inner surface of the face plate by coating, and a shadow mask having a large number of electron beam apertures is placed opposite to the fluorescent layer. The neck portion houses an electron gun which emits three electron beams. In the funnel portion, an internal magnetic shield made of a substantially quadrangular pyramid-shaped frame structure is disposed inside the color cathode-ray tube, while a deflection coil is disposed outside the same tube.
In this case, the internal magnetic shield is disposed for the purpose that three electron beams emitted from the electron gun are prevented from being affected by terrestrial magnetism. If the internal magnetic shield does not have a sufficient effect of shielding terrestrial magnetism, the three electron beams are affected by terrestrial magnetism to be caused to slightly deviate from the original electron beam path, with the result that the display image of the color cathode-ray tube is deteriorated in color purity and suffered from color contamination.
FIGS. 5A to 5C show an example of construction of a conventional internal magnetic shield used in a known color cathode-ray tube, and FIG. 5A is a perspective view, FIG. 5B is a top view and FIG. 5C is a side view.
As shown in FIGS. 5A to 5C, a known internal magnetic shield is made of a substantially quadrangular pyramid-shaped frame member 40 made up of two long side walls 41A, 41B and two short side walls 42A, 42B. The internal magnetic shield has a substantially rectangular first opening 43 with a smaller diagonal dimension at one end adjacent to an electron gun and than that of a larger diagonal dimension of a substantially rectangular second opening 44 at the other end adjacent to a shadow mask. The two long side walls 41A, 41B are formed in the portions thereof adjacent to the first opening 43 with substantially V-shaped notches 43A, 43B having a maximum depth cxe2x80x2, respectively.
When the frame member 40 is disposed inside the funnel portion, an edge portion 45 of the second opening 44 is fitted to a support frame mounted on the side wall of the panel portion together with the peripheral portion of the shadow mask. In this case, the substantially rectangular first opening 43 of smaller diagonal dimension faces an electron gun and the substantially rectangular second opening 44 of larger diagonal dimension faces the shadow mask so as to allow three electron beams emitted from the electron gun to pass through the inside of the frame member 40 and strike a fluorescent layer through one of electron beam apertures of the shadow mask.
In the meantime, the substantially V-shaped notches 43A, 43B formed in the two long side walls 41A, 41B are provided for regulating the path for the electron beam passing through the inside of the frame member 40. By selecting the maximum depth cxe2x80x2 of the substantially V-shaped notches 43A, 43B, the amount of terrestrial magnetism converging on the two long side walls 41A, 41B and the two short side walls 42A, 42B is controlled. Incidentally, the substantially V-shaped notches 43A, 43B may be formed in the two short side walls 42A, 42B instead of being formed in the two long side walls 41A, 41B, in which case the same performance can be attained as well.
In such internal magnetic shield, however, if the maximum depth cxe2x80x2 of the substantially V-shaped notches 43A, 43B is increased for the purpose of appropriate regulation of the electron beam path, although the electron beam path can be regulated, there arises a problem that the effective area of the two long side walls 41A, 41B or the two short side walls 42A, 42B is reduced correspondingly to an increment of depth of the substantially V-shaped notches 43A, 43B, resulting in deterioration of the total shielding effect of the internal magnetic shield.
The present invention aims to solve the above problem.
It is an object of the present invention to provide a color cathode-ray tube having an internal magnetic shield which is capable of appropriately regulating an electron beam path even if the maximum depth of a substantially V-shaped notch is made small lest a total shielding effect should be deteriorated.
To achieve the above object, there is provided according to the present invention a color cathode-ray tube having an internal magnetic shield, which comprises at least a fluorescent layer formed on an inner surface of a face plate of a panel portion, a shadow mask disposed opposite to the fluorescent layer, an electron gun housed in a neck portion, and the internal magnetic shield disposed in a funnel portion and made of a substantially quadrangular pyramid-shaped frame member which has a substantially rectangular first opening of small diagonal dimension at one end adjacent to the electron gun and a substantially rectangular second opening of large diagonal dimension at the other end adjacent to the shadow mask, and creased lines formed between corresponding corners of the first and second openings, wherein each of the creased lines of the internal magnetic shield is formed in such a manner that an end of an imaginary line extension of the creased line adjacent to the second opening is located on a projected plane parallel to the second opening at a point shifted by a predetermined length from the corresponding corner of the second opening in the direction of a side of the second opening, and a segment is made by connecting a predetermined point on a line connecting between the end of the imaginary line extension and the corresponding corner of the first opening to the corresponding corner of the second opening so as to form a part of the creased line adjacent to the second opening, thereby adjusting the area ratio of side faces of the internal magnetic shield.
Preferably, the ends of the imaginary line extensions of the creased lines adjacent to the substantially rectangular second opening are located on the projected plane at the points shifted by a predetermined length from the corners in the direction of long side when the fluorescent layer is made of a large number of phosphor dots.
It is also preferred that the ends of the imaginary line extensions of the creased lines adjacent to the substantially rectangular second opening are located on the projected plane at the points shifted by a predetermined length from the corners in the direction of short side when the fluorescent layer is made of a large number of phosphor stripes.
According to the present invention, the ends of the imaginary line extensions of the creased lines adjacent to the second opening are located at the points shifted by a predetermined length from the corners in the direction of side for the purpose that the ratio of the effective area of the two long side walls to the effective area of the two short side walls is adjusted by selecting the predetermined length instead of the known means of adjusting the maximum depth of the substantially V-shaped notches formed in the two long side walls or two short side walls. And accordingly, even if the maximum depth of the substantially V-shaped notches is so selected as to become small, it is possible to appropriately regulate the electron beam path, and moreover the total shielding effect is not deteriorated.
In the present invention, the ends of the imaginary line extensions of the creased lines adjacent to the second opening are the points located on the sides of the second opening on the projection plane when the internal magnetic shield is projected on a plane parallel to the opening of the magnetic shield.